The proposed projects involve making single-unit recording studies in the awake monkey to test a simple cortical circuit model of attention. If supported empirically, the model promises to provide a major simplification in our understanding of the neural mechanisms of spatial attention. At the end of the proposed course of research, we expect to have achieved important and profoundly simplifying insights into the neural mechanisms of attention. We will have characterized key contrast-dependent response properties of V4, which itself will constitute an important step forward (Aim 1). We will have tested whether these contrast-dependent response properties in Area V4 mirror those of V1 (Aim 1). Understanding any differences that may exist between contrast modulation in V4 and V 1 is important. To the extent that they are similar, this will allow us to establish a mechanistic link between these two important visual processing areas, and to broaden the significance of models of processing in V 1. We will have tested whether spatial attention modulates center-surround interactions (Aim 2), which has major ramifications for our understanding of earlier studies. For example, single-unit recording studies in V2, V4, MT and TEO have established that when two stimuli appear together within a neuron's classical RF, the response is driven preferentially by the attended stimulus. The proposed studies will determine whether this pattern occurs as a result of biasing suppressive center-surround interactions at earlier stages. If so, this will support the simple model advanced in this proposal. If not, this would strongly suggest that the selection occurs at the first stage where RFs are large enough to encompass the two stimuli. Finally, we will have measured the relationship between contrast-dependent center-surround modulations and attentional selection (Aim 3), providing the strongest test yet of the proposal that attention operates by increasing the effective contrast of the attended stimulus. Taken together, the proposed research promises to provide a much deeper understanding of the cortical circuits that mediate attentional selection, than has been achieved to date.